Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation.
Photosensitive optoelectronic devices convert electromagnetic radiation into an electrical signal or electricity. Solar cells, also called photovoltaic (“PV”) devices, are a type of photosensitive optoelectronic devices that are specifically used to generate electrical power. Photoconductor cells are a type of photosensitive optoelectronic device that are used in conjunction with signal detection circuitry which monitors the resistance of the device to detect changes due to absorbed light. Photodetectors, which may receive an applied bias voltage, are a type of photosensitive optoelectronic device that are used in conjunction with current detecting circuits which measures the current generated when the photodetector is exposed to electromagnetic radiation.
These three classes of photosensitive optoelectronic devices maybe distinguished according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector may have a rectifying junction and is usually but not always operated with a bias.
When electromagnetic radiation of an appropriate energy is incident upon an organic semiconductor material, a photon can be absorbed to produce an excited molecular state. In organic photoconductive materials, the excited molecular state is generally believed to be an “exciton,” i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. An exciton can have an appreciable life-time before geminate recombination (“quenching”), which refers to the original electron and hole recombining with each other (as opposed to recombination with holes or electrons from other pairs). To produce a photocurrent, the electron-hole forming the exciton is typically separated at a rectifying junction.
In the case of photosensitive devices, the rectifying junction is referred to as a photovoltaic heterojunction. Types of organic photovoltaic heterojunctions include a donor-acceptor heterojunction formed at an interface of a donor material and an acceptor material, and a Schottky-barrier heterojunction formed at the interface of a photoconductive material and a metal.
In the context of organic materials, the terms “donor” and “acceptor” refer to the relative positions of the Highest Occupied Molecular Orbital (“HOMO”) and Lowest Unoccupied Molecular Orbital (“LUMO”) energy levels of two contacting but different organic materials. If the HOMO and LUMO energy levels of one material in contact with another are lower, then that material is an acceptor. If the HOMO and LUMO energy levels of one material in contact with another are higher, then that material is a donor. It is energetically favorable, in the absence of an external bias, for electrons at a donor-acceptor junction to move into the acceptor material.
As used herein, a first HOMO or LUMO energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. A higher HOMO energy level corresponds to an ionization potential (“IP”) having a smaller absolute energy relative to a vacuum level. Similarly, a higher LUMO energy level corresponds to an electron affinity (“EA”) having a smaller absolute energy relative to vacuum level. On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material.
After absorption of a photon in the material creates an exciton, the exciton dissociates at the rectifying interface. A donor material will transport the hole, and an acceptor material will transport the electron.
A significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. In the context of organic photosensitive devices, a material that conducts preferentially by electrons due to high electron mobility may be referred to as an electron transport material. A material that conducts preferentially by holes due to a high hole mobility may be referred to as a hole transport material. A layer that conducts preferentially by electrons, due to mobility and/or position in the device, may be referred to as an electron transport layer (“ETL”). A layer that conducts preferentially by holes, due to mobility and/or position in the device, may be referred to as a hole transport layer (“HTL”). Preferably, but not necessarily, an acceptor material is an electron transport material and a donor material is a hole transport material.
How to pair two organic photoconductive materials to serve as a donor and an acceptor in a photovoltaic heterojunction based upon carrier mobilities and relative HOMO and LUMO levels is well known in the art, and is not addressed here.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substitute does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule.” In general, a small molecule has a defined chemical formula with a molecular weight that is the same from molecule to molecule, whereas a polymer has a defined chemical formula with a molecular weight that may vary from molecule to molecule. As used herein, “organic” includes metal complexes of hydrocarbyl and heteroatom-substituted hydrocarbyl ligands.
An organic photosensitive device comprises at least one photoactive region in which light is absorbed to form an exciton, which may subsequently dissociate into an electron and a hole. The photoactive region will typically comprise a donor-acceptor heterojunction, and is a portion of a photosensitive device that absorbs electromagnetic radiation to generate excitons that may dissociate in order to generate an electrical current.
Organic photosensitive devices may incorporate electron blocking layers (EBLs). EBLs are described in U.S. Pat. No. 6,451,415 to Forrest et al., which is incorporated herein by reference for its disclosure related to EBLs. EBLs (among other things) reduce quenching by preventing excitons from migrating out of the donor and/or acceptor materials. It is generally believed that the EBLs derive their exciton blocking property from having a LUMO-HOMO energy gap substantially larger than that of the adjacent organic semiconductor from which excitons are being blocked. Thus, the confined excitons are prohibited from existing in the EBL due to energy considerations. While it is desirable for the EBL to block excitons, it is not desirable for the EBL to block all charge. However, due to the nature of the adjacent energy levels, an EBL may block one sign of charge carrier. By design, an EBL will exist between two other layers, usually an organic photosensitive semiconductor layer and an electrode or a charge transfer layer. The adjacent electrode or charge transfer layer will be in context either a cathode or an anode. Therefore, the material for an EBL in a given position in a device will be chosen so that the desired sign of carrier will not be impeded in its transport to the electrode or charge transfer layer. Proper energy level alignment ensures that no barrier to charge transport exists, preventing an increase in series resistance.
It should be appreciated that the exciton blocking nature of a material is not an intrinsic property of its HOMO-LUMO energy gap. Whether a given material will act as an exciton blocker depends upon the relative HOMO and LUMO energy levels of the adjacent organic photosensitive material, as well upon the carrier mobility and carrier conductivity of the material. Therefore, it is not possible to identify a class of compounds in isolation as exciton blockers without regard to the device context in which they may be used. However, one of ordinary skill in the art may identify whether a given material will function as an exciton blocking layer when used with a selected set of materials to construct an organic PV device. Additional background explanation of EBLs can be found in U.S. patent application Ser. No. 11/810,782 of Barry P. Rand et al., published as 2008/0001144 A1 on Jan. 3, 2008, the disclosure of which is incorporated herein by reference, and Peumans et al., “Efficient photon harvesting at high optical intensities in ultrathin organic double-heterostructure photovoltaic diodes,” Applied Physics Letters 76, 2650-52 (2000).
The terms “electrode” and “contact” are used interchangeably herein to refer to a layer that provides a medium for delivering photo-generated current to an external circuit or providing a bias current or voltage to the device. Electrodes may be composed of metals or “metal substitutes.” Herein the term “metal” is used to embrace both materials composed of an elementally pure metal, and also metal alloys which are materials composed of two or more elementally pure metals. The term “metal substitute” refers to a material that is not a metal within the normal definition, but which has the metal-like properties such as conductivity, such as doped wide-bandgap semiconductors, degenerate semiconductors, conducting oxides, and conductive polymers. Electrodes may comprise a single layer or multiple layers (a “compound” electrode), may be transparent, semi-transparent, or opaque. Examples of electrodes and electrode materials include those disclosed in U.S. Pat. No. 6,352,777 to Bulovic et al., and U.S. Pat. No. 6,420,031, to Parthasarathy, et al., each incorporated herein by reference for disclosure of these respective features. As used herein, a layer is said to be “transparent” if it transmits at least 50% of the ambient electromagnetic radiation in a relevant wavelength.
The functional components of organic photosensitive devices are usually very thin and mechanically weak, and therefore the devices are typically assembled on the surface of a substrate. The substrate may be any suitable substrate that provides desired structural properties. The substrate may be flexible or rigid, planar or non-planar. The substrate may be transparent, translucent or opaque. Rigid plastics and glass are examples of preferred rigid substrate materials. Flexible plastics and metal foils are examples of preferred flexible substrate materials.
Organic donor and acceptor materials for use in the photoactive region may include organometallic compounds, including cyclometallated organometallic compounds. The term “organometallic” as used herein is as generally understood by one of ordinary skill in the art and as given, for example, in Chapter 13 of “Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A. Tarr, Prentice Hall (1999).
Organic layers may be fabricated using vacuum deposition, spin coating, organic vapor-phase deposition, organic vapor jet deposition, inkjet printing and other methods known in the art.
Donor and acceptor layers may meet at a bilayer, forming a planar heterojunction. A hybrid or mixed heterojunction comprises a mixture of donor and acceptor materials, arranged between layers of donor material and acceptor material. A bulk heterojunction, in the ideal photocurrent case, has a single continuous interface between the donor material and the acceptor material, although multiple interfaces typically exist in actual devices. Mixed and bulk heterojunctions can have multiple donor-acceptor interfaces as a result of having plural domains of material. Domains that are surrounded by the opposite-type material (e.g., a domain of donor material surrounded by acceptor material) may be electrically isolated, such that these domains do not contribute to photocurrent. Other domains may be connected by percolation pathways (continuous photocurrent pathways), such that these other domains may contribute to photocurrent. The distinction between a mixed and a bulk heterojunction lies in degrees of phase separation between donor and acceptor materials. In a mixed heterojunction, there is very little or no phase separation (the domains are very small, e.g., less than a few nanometers), whereas in a bulk heterojunction, there is significant phase separation (e.g., forming domains with sizes of a few nanometers to 100 nm). Isolated carbon nanotubes act as individual domains, and if present in sufficient concentrations may give rise to percolation pathways.
Small-molecule mixed heterojunctions may be formed, for example, by co-deposition of the donor and acceptor materials using vacuum deposition or vapor deposition. Small-molecule bulk heterojunctions may be formed, for example, by controlled growth, co-deposition with post-deposition annealing, or solution processing. Polymer mixed or bulk heterojunctions may be formed, for example, by solution processing of polymer blends of donor and acceptor materials.
In general, planar heterojunctions have good carrier conduction, but poor exciton dissociation; a mixed layer has poor carrier conduction and good exciton dissociation, and a bulk heterojunction has good carrier conduction and good exciton dissociation, but may experience charge build-up at the end of the material “cul-de-sacs,” lowering efficiency. Unless otherwise stated, planar, mixed, bulk, and hybrid heterojunctions may be used interchangeably as donor-acceptor heterojunctions throughout the embodiments disclosed herein.
The photoactive region may be part of a Schottky-barrier heterojunction, in which a photoconductive layer forms a Schottky contact with a metal layer. If the photoconductive layer is an ETL, a high work function metal such as gold may be used, whereas if the photoconductive layer is an HTL, a low work function metal such as aluminum, magnesium, or indium may be used. In a Schottky-barrier cell, a built-in electric field associated with the Schottky barrier pulls the electron and hole in an exciton apart. Generally, this field-assisted exciton dissociation is not as efficient as the dissociation at a donor-acceptor interface.
The devices may be connected to a resistive load which consumes or stores power. If the device is a photodetector, the device is connected to a current-detecting circuit which measures the current generated when the photodetector is exposed to light, and which may apply a bias to the device (as described for example in Published U.S. Patent Application 2005-0110007 A1, published May 26, 2005 to Forrest et al.). If the rectifying junction is eliminated from the device (e.g., using a single photoconductive material as the photoactive region), the resulting structure may be used as a photoconductor cell, in which case the device is connected to a signal detection circuit to monitor changes in resistance across the device due to the absorption of light. Unless otherwise stated, each of these arrangements and modifications may be used for the devices in each of the drawings and embodiments disclosed herein.
An organic photosensitive optoelectronic device may also comprise transparent charge transfer layers, electrodes, or charge recombination zones. A charge transfer layer may be organic or inorganic, and may or may not be photoconductively active. A charge transfer layer is similar to an electrode, but does not have an electrical connection external to the device and only delivers charge carriers from one subsection of an optoelectronic device to the adjacent subsection. A charge recombination zone is similar to a charge transfer layer, but allows for the recombination of electrons and holes between adjacent subsections of an optoelectronic device. A charge recombination zone may include semi-transparent metal or metal substitute recombination centers comprising nanoclusters, nanoparticles, and/or nanorods, as described for example in U.S. Pat. No. 6,657,378 to Forrest et al.; Published U.S. Patent Application 2006-0032529 A1, entitled “Organic Photosensitive Devices” by Rand et al., published Feb. 16, 2006; and Published U.S. Patent Application 2006-0027802 A1, entitled “Stacked Organic Photosensitive Devices” by Forrest et al., published Feb. 9, 2006; each incorporated herein by reference for its disclosure of recombination zone materials and structures. A charge recombination zone may or may not include a transparent matrix layer in which the recombination centers are embedded. A charge transfer layer, electrode, or charge recombination zone may serve as a cathode and/or an anode of subsections of the optoelectronic device. An electrode or charge transfer layer may serve as a Schottky contact.
For additional background explanation and description of the state of the art for organic photosensitive devices, including their general construction, characteristics, materials, and features, U.S. Pat. Nos. 6,972,431, 6,657,378 and 6,580,027 to Forrest et al., and U.S. Pat. No. 6,352,777 to Bulovic et al., are incorporated herein by reference in their entireties.
The discovery in 1992 of photoinduced charge transfer between conjugated polymers and fullerenes (N. S. Sariciftci et al., Proc. SPLE, 1852:297-307 (1993)) has inspired a great deal of research into the possible use of fullerenes in photovoltaic and photoelectric devices. This led to the fabrication of several photovoltaic systems that employ a combination of polymer and fullerenes. It has been found that fullerenes can be susceptible to photooxidation. The observation of photoinduced electron transfer at a multi-wall carbon nanotube-conjugated polymer interface (H. Ago et al., Phys. Rev. B, 61:2286 (2000)) has inspired attempts to use carbon nanotubes (CNTs) and in particular single-walled carbon nanotubes (SWNTs) as electron acceptor materials in photovoltaic devices.
The first reported use of CNTs as electron acceptors in a bulk-heterojunction photovoltaic cell was a blend of SWNTs with polythiophenes, in which an increase in photocurrent of two orders of magnitude was observed (E. Kymakis, G. A. J. Amaratunga, Appl. Phys, Lett. 80:112 (2002)). In 2005, a photovoltaic effect was observed in an isolated SWNT illuminated with 1.5 μm (0.8 eV) radiation (J. U. Li, Appl. Phys. Lett. 87:073101 (2005)).
Kymakis (E. Kymakis and G. Amaratunga, Rev. Adv. Mat. Sci. 10:300-305 (2005,) has described the use of carbon nanotubes as electron acceptors in a polymeric photovoltaic system based on poly (3-octylthiophene). In this system, the nanotubes serve as electron acceptors and electron conductors; the photocurrent declines at CNT concentrations greater than about 1%, and the authors concluded that the nanotubes do not contribute to the photocurrent.
Ajayan et al., (U.S. Patent Application Publication No. 2006/0272701) have described the use of SWNTs as the electron-transporting component in a photovoltaic device, using covalently attached organic dyes as the photo-responsive component. More recently, Mitra et al., have similarly employed SWNTs as the electron-transporting component in a photovoltaic device based on C60-organic semiconductor heterojunctions (C. Li. et al., J. Mater. Chem. 17, 2406 (2007); C. Li and S. Mitra, Appl. Phys. Lett. 91, 253112 (2007)). These devices employ SWNTs as electron-accepting and electron-conducting elements. Previous workers have noted that the metallic SWNTs in these devices provide short-circuit pathways for the recombination of holes and electrons, and have speculated that the devices would be more efficient if isolated semiconducting SWNTs were employed (E. Kymakis et al., J. Phys. D: Appl. Phys. 39, 1058-1062 (2006); M. Vignali et al., http://re.jrc.cec.eu.int/solarec/publications/paris_polymer.pdf (undated)). However, the use of semiconducting SWNTs in such designs employ the SWNTs as electron-accepting and electron-conducting elements only, rather than as sources of photogenerated excitons. The existing and proposed devices do not take advantage of the photoelectric properties of semiconducting SWNTs.
Currently, all synthetic methods for growing SWNTs result in heterogeneous mixtures of SWNTs that vary in their structural parameters (length, diameter, and chiral angle), and consequently have variations in their electronic and optical properties (e.g., conductivity, electrical band gap, and optical band gap) (M. S. Arnold, A. A. Green, J. F. Hulvat et al., NatureNanotech. 1(1), 60 (2006); M. S. Arnold, S. I. Stupp, and M. C. Hersam, Nano Letters 5 (4), 713 (2005); R. H. Baughman, A. A. Zakhidov, and W. A. de Heer, Science 297 (5582), 787 (2002). All reported CNT-based photovoltaic devices reported to date employ these mixtures.
Recent advances include fabrication methods for CNT thin films on various substrates such as (polyethylene terephthalate (PET), glass, polymethylmethacrylate) (PMMA), and silicon (Y. Zhou, L. Hu, G. Griiner, Appl. Phys. Lett. 88:123109 (2006)). The method combines vacuum filtration generation of CNT mats with a transfer-printing technique, and allows controlled deposition and patterning of large area, highly conducting CNT films with high homogeneity. Such films are a potential alternative to the commonly-used hole-collecting electrode material, indium-tin oxide (ITO), which is expensive and remains incompatible with roll-to-roll fabrication processing.
The properties of carbon nanotubes are influenced mostly by the diameter of the tube and the degree of twist. Both aligned tubes and tubes with a twist can be metallic or semiconducting, depending on whether the energy states in the circumferential direction pass through what is termed a Fermi point. At Fermi points, the valence and conduction bands meet, which allows for conduction in the circumferential direction of the tube. Tubes that have the correct combination of diameter and chirality will possess a set of Fermi points around the perimeter of their grid structures throughout the length of the tube. These tubes will show metallic like conduction. If the diameter and chirality do not generate a set of Fermi points, the tube will exhibit semiconducting behavior (P. Avouris, Chemical Physics, 281: 429-445 (2002)).
In addition to Fermi point matchups, the cylindrical shape and diameter of the tube affects electron transport through the way in which quantum states exist around the tube perimeter. Small diameter tubes will have a high circumferential band gap with a low number of energy states available. As the tube diameter increases, the number of energy states increases and the circumferential band gap decreases. In general, the band gap is inversely proportional to the tube diameter.
Furthermore, the wave properties of electrons are such that standing waves can be set up radially around a carbon nanotube. These standing waves, the lack of conduction states in small diameter tubes, and the monolayer thickness of the graphite sheet, combine to inhibit electron motion around the tube perimeter and force electrons to be transported along the tube axis.
If a Fermi point matchup is present, however, electron transport can occur around the tube perimeter, in addition to axial conduction, allowing for increased transport options of the electron and metallic conduction characteristics. As the tube diameter increases, more energy states are allowed around the tube perimeter and this also tends to lower the band gap. Thus, when only axial conduction is allowed, the tube exhibits semiconducting behavior. When both axial and circumferential conduction are allowed, the tube exhibits metallic conduction.
The power output of existing organic photovoltaic devices is not yet competitive with traditional silicon-based photovoltaic devices. In addition to being less efficient and like other thin-film approaches, they are susceptible to oxidative degradation when exposed to air, and need encapsulation. Given the cost and fragility of silicon solar cells, and the promise of easily-fabricated and inexpensive organic equivalents, there remains a need for more efficient and more stable organic photovoltaic and photoelectric devices. Also, because of organic materials' poor sensitivity to IR and near-IR radiation, there remains a need for organic photovoltaic materials capable of efficiently producing excitons upon irradiation by IR and NIR radiation.
Semiconducting CNTs, despite their strong near-IR band gap absorption, have only had limited impact as the optically absorptive components of optoelectronic devices because of the strong binding energy of photogenerated electron-hole pairs.